Add a human-friendly HTML template for mentions

[acelaya]

Depends on #9357

Closes #9355
Closes #9341

This PR brings the HTML template used in LMS for the email digest, and reuses the same structure for @mentions emails, but trying to more or less display the information from the original email design: https://www.figma.com/design/pwzOVmALQqBB8OFDeYTMTd/%40Mentions?node-id=331-2151&p=f&t=7ogj2TOnBfTnUDoC-0

Considerations and out of scope

• When we set annotation.text_rendered, there's some markdown rendering and HTML sanitization that ends up stripping all data attributes from mention tags. This only affects how they are styled in the email, but I'll try address this separately. EDIT: To be addressed in Keep mention attributes when filtering annotation markdown #9361
• The mention_notification unit test uses a jinja environment to test for potential syntax errors in the template. I had to mock one global jinja function there as a workaround, but I'll try to add a proper functional test as well. See the slack discussion for details.
• The email is currently lacking unsubscribe or survey links. We still need to clarify those.

Test steps

1. Checkout this branch.
2. Create an annotation mentioning some user.
3. See that the mail folder in h contains a file corresponding to the email notification for that mention.
4. Open that file in a mail client, like Thunderbird, and verify it includes the information and links for said annotation.